This invention relates to a method of manufacturing a colour picture tube, more particularly to a film forming process performed before vapour deposition of a metallic reflecting film of a fluorescent screen.
The fluorescent screen of a colour picture tube is generally manufactured by the steps of coating a slurry of a photosensitive phosphor on the inner surface of the face plate of the tube, drying the coated film, exposing the film to light through an apertured mask such as a shadow mask, developing the exposed film with water, drying the developed film thus shaping the phosphor in the form of dots or stripes, and repeating the above-described process steps for three primary colours, thereby arranging phosphors for emanating green, blue and red colours in the form of dots or stripes of a predetermined pattern. Then, a film of acrylic resin is formed on the dots or stripes of the phosphors of three colours prior to vapour deposition of a metallic reflecting film which is provided for the purpose of improving the brightness of the fluorescent screen. According to one method of forming the acrylic resin film, an acrylic resin emulsion is used. In such emulsion film forming process, a film forming liquid is prepared by adding a suitable amount of a boric acid ester of a polyvinyl alcohol, hydrogen peroxide solution, colloidal silica, etc, to an acrylic resin emulsion. Then, the film forming liquid is coated on the phosphor dots or stripes, and then heated and dried to form an acrylic resin film. Thereafter, aluminum or the like is vapour deposited on the film to form a metallic reflecting film. The face plate formed with the fluorescent screen is then heated at a high temperature in a baking furnace to subject such an organic substance as the acrylic resin film to a pyrolysis for decomposing it into gases which are then removed. In performing the emulsion film forming process, it is important to form a metallic reflecting film capable of effficiently reflecting the light emanated by the phosphors so as to improve the brightness of the fluorescent screen. If the concentration of the acrylic resin in the film forming liquid were too low, the surface 3a of the resulting acrylic resin film would become irregular on account of the irregular surface of the phosphor film, as shown in FIG. 1a. On the other hand, if the concentration of the acrylic resin in the film forming liquid were too high, the surface 3b of the resulting, acrylic resin film would be flat as shown in FIG. 1b. In FIGS. 1a and 1b, reference numeral 1 designates a face plate, 2 particles of the phosphors, and 3a and 3b the surface of the acrylic resin film. However, when the surface of the acrylic resin film is flat and smooth and hence the surface of the metallic reflecting film is flat and smooth and free from pin holes, the gases formed by the pyrolysis of the acrylic resin during the baking step are difficult to remove because the metallic reflecting film contains less number of pin holes. As a result, the metallic reflecting film 4 would bulge as shown at 5 in FIG. 2, in which 1 designates the face plate and 6 a phosphor film. Bulging of the metallic reflecting film is generally prevented by adjusting the concentration of the acrylic resin of the flim forming liquid or the concentration of the additives including boric acid ester of a polyvinyl alcohol and hydrogen peroxide solution so that a suitable number of pin holes or cracks are formed in the metallic reflecting film vapour-deposited on the surface of the acrylic resin film. However, it is of course desirable that the metallic reflecting film should contain a small number of pin holes and cracks for the purpose of improving the brightness of the picture. Thus it is important to select the number of the pin holes and cracks such that they can efficiently prevent bulging of the metallic reflecting film due to the gas produced by organic substances while preserving the desired brightness.